Medium density ethylene polymers, a process to prepare these polymers and use of carbonyl group containing chain transfer agents in this process

ABSTRACT

The present invention relates to an ethylene homo or copolymer having a density of between 0.923 and 0.935 g/cm 3 , having a molecular weight distribution M w /M n  of between 3 and 10, and comprising from 0.10 to 0.50 wt. percent of units derived from a carbonyl group containing compound, based on the total weight of the homo or copolymer. In addition, the invention relates to a free radical initiation polymerization process for the preparation of medium density ethylene polymers or copolymers, comprising reacting ethylene and optionally one or more comonomers at a high pressure, conveniently between 1600 and 4000 kg/cm 2 , and at temperatures of about 150-330° C. in a reactor system consisting of at least one autoclave reactor or of a combination of autoclave and tubular reactors, in the presence of free radical initiators and a carbonyl group containing compound. Finally, the invention relates to carbonyl group containing chain transfer agents to obtain improved polymer processing and performance properties in flat die extrusion processes and applications.

The present invention relates to medium density ethylene homo andcopolymers, and more in particular to medium density LDPE-type (lowdensity polyethylene) resins. In addition, the present invention relatesto a high pressure ethylene homo or copolymerization process and to theuse of carbonyl group containing chain transfer agents such as ketonesor aldehydes, and especially methyl ethyl ketone (MEK) orpropionaldehyde, in the polymerization process.

Medium density ethylene homo and copolymers, which polymers have adensity of between 0.925 and 0.935 g/cm³, are well known in the art.These known polymers can, for instance, be prepared in high pressureradical initiated polymerization processes, wherein a wide variety ofdifferent chain transfer agents can be used.

Chain transfer agents or telogens are used to control the melt flowindex in a polymerization process. Chain transfer involves thetermination of growing polymer chains, thus limiting the ultimatemolecular weight of the polymer material. Chain transfer agents aretypically hydrogen atom donors that will react with a growing polymerchain and stop the polymerization reaction of said chain. These agentscan be of many different types, from saturated hydrocarbons orunsaturated hydrocarbons to aldehydes, ketones or alcohols. Bycontrolling the concentration of the selected chain transfer agent, onecan control the length of polymer chains, and, hence, the weight averagemolecular weight, M_(w). The melt flow index (MFI or I₂) of a polymer,which is related to M_(w), is controlled in the same way.

After the donation of a hydrogen atom, the chain transfer agent forms aradical which can react with the monomers, or with already formedoligomers or polymers, to start a new polymer chain. This means that anyfunctional groups present in chain transfer agents, for instancecarbonyl groups of aldehydes and ketones, will be introduced in thepolymer chains.

A large number of chain transfer agents, for example propylene and1-butene which have an olefinically unsaturated bond, can also beincorporated in the polymer chain, themselves, via a copolymerizationreaction. This generally leads to the formation of short chain branchingof respectively methyl and ethyl groups, which lowers the density of thepolymers obtained.

In many processes, two types of chain transfer agents are used in orderto control both the M_(w) (and MFI) and the density of the polymersprepared.

The melt flow index of the product polymer can be controlled by varyingthe amount of chain transfer agent present during the polymerization,usually by mixing varying amounts of transfer agent with the monomer orthe mixture of monomers prior to polymerization. Polymers produced inthe presence of chain transfer agents are modified in a number ofphysical properties such as processability, optical properties such ashaze and clarity, density, stiffness, yield point, film draw and tearstrength.

The use of carbonyl group containing compounds, and especially ketonesor aldehydes, such as MEK and propionaldehyde, as chain transfer agentand molecular weight regulator in high pressure polyethylene(co)polymerization processes using autoclave and tubular reactors, iswell known for at least 30 years.

DE-OS-19 08 964 teaches an ethylene homopolymer preparation processusing two tubular reactors in series, and using organic peroxides asradical initiators. The reaction temperatures and pressures are withinthe ranges of 250-340° C. and 1500-4000 kg/cm². As an example of asuitable polymerization controller MEK is mentioned. The polymer productis said to have a narrow molecular weight distribution (MWD), a goodtransparency and a good gloss.

In an article in the Journal of Polymer Science: Part A-1; vol 4,881-900 (1966), Mortimer describes the use of, a.o., aldehydes such aspropionaldehyde, and ketones such as MEK, as chain transfer agents in ahigh-pressure free-radical polymerization process.

U.S. Pat. No. 3,129,212 teaches that chemical modifiers such aspropylene and MEK can be used to prepare polyethylenes having a narrowMWD and a high density.

Also, U.S. Pat. No. 3,334,081 teaches that chain transfer agents can beused to increase the density of solid polyethylenes. Among various chaintransfer agents, MEK and aldehydes are mentioned.

In U.S. Pat. No. 3,317,504, a high pressure ethylene polymerizationprocess is described using a tubular reactor and, a.o., MEK as transferagent. The polymers obtained were said to have a density of up to 0.940g/cm³.

East German patent 108,546 teaches a high-pressure ethylene homo-, co-or terpolymerization using free radical forming initiators. Aspolymerization regulators, among other, MEK and propylene are mentioned,as well as a combination thereof. This document teaches the addition ofchain regulators in two tubular reactor zones to reduce molecular weightfluctuations, which leads to better workability and better film formingproperties.

U.S. Pat. No. 3,293,233 describes that polymers of ethylene can beobtained when certain chain transfer agents are employed in ahigh-pressure polymerization process. Among the huge amount of chaintransfer agents, MEK is explicitly referred to.

In U.S. Pat. No. 3,691,145 a high pressure polyethylene preparationprocess is described using polymerization regulators. The polymerizationregulators used may be “those usual in the art, such as alkanes, alkenesof more than two carbon atoms, alcohols, ethers, aldehydes, ketones ormixtures of such substances”. Reference is made to the above-mentionedarticle of Mortimer.

U.S. Pat. No. 3,917,577 describes a continuous process for theproduction of ethylene homopolymers in a tubular reactor having at leasttwo reaction zones. Ethylene, initiator and regulator are continuouslyintroduced at the beginning of each reaction zone. It is the object ofthis patent to provide a multistage process giving a polyethylene havinga narrower MWD. In order to achieve this, one has to use thepolymerization regulators having a high C value as described byMortimer. MEK and propionaldehyde are mentioned among the preferredregulators.

U.S. Pat. No. 4,076,919 teaches to use conventional regulators such aspropylene and MEK in a high pressure two-zone tubular reactor ethylenepolymerization or co polymerization process.

U.S. Pat. No. 4,085,266 teaches the same chain transfer agents in atwo-zone autoclave ethylene copolymerization process. In the top zone ofthe autoclave reactor ethylene is polymerized at a pressure of between1,000 and 1,800 kg/cm² and at a relatively low temperature of between130 and 200° C., followed by a reaction in the bottom zone at a pressurein the same range and at a high temperature of between 220 and 280° C.

In U.S. Pat. No. 4,123,600, a high pressure LDPE preparation process isdescribed using a battery of two or more autoclave reactors, which areoperated in the same way as the autoclave reactor described in theprevious paragraph.

In the high-pressure polymerization process described in U.S. Pat. No.4,168,355, the melt index of ethylene homo and copolymers is taught tobe affected in a conventional way by the addition of a chain transferagent.

U.S. Pat. No. 4,988,781 teaches the production of an improvedhomogeneous interpolymer of ethylene and an α-olefinically unsaturatedcarboxylic acid or ester in a stirred autoclave. The polymer product issaid to have a substantially narrow MWD, appreciably reduced levels oflong-chain branching, substantially improved extrusion stability, andappreciably improved draw-down. The improvements are obtained by using atelogenic modifier. MEK is referred to as an especially preferredtelogen.

When in these known processes using carbonyl group containing compounds,such as MEK or propionaldehyde, as chain transfer agent (CTA) polymerproducts having a medium density are prepared, the obtained polymerproducts usually have a narrow molecular weight distribution of lessthan 3.0 and contain reduced amounts of high molecular weight fractions.This is due to the high reactivity of the carbonyl group containingchain transfer agent used. In addition, these chain transfer agents havenegligible copolymerization possibilities resulting in polymers havingmedium densities. Short chain branches due to copolymerization of theCTA are not or hardly formed.

Such polymer products having a narrow MWD are very suitable for theproduction of high clarity LDPE blown films, however not for cast filmor extrusion coating applications, which require polymers having broademolecular weight distributions.

Further, the known processes, which in practice essentially make use ofa tubular reactor, have relatively low ethylene conversions of maximallyabout 16-18 percent, because in order to obtain preferred medium densityLDPE's, having densities of between 0.925 and 0.935 g/cm³ the reactortemperature range has to be kept relatively low at values of about100-260° C., depending on the reactor configuration. In known autoclaveprocesses, broad molecular weight products for cast film and extrusioncoating can be obtained. However, these autoclave processes normallyrequire relatively low pressures, which means that only lowconcentrations of ketones or aldehydes can be used.

It is a first object of the present invention to provide ethylene homoand copolymers which have a medium density, while having a relativelybroad molecular weight distribution.

It is a second object of the present invention to provide ethylene homoand copolymers for cast films and extrusion coating applications.

It is a third object of the present invention to provide ethylene homoand copolymers having a relatively high content of units derived fromcarboxyl group containing chain transfer agents, such as ketones andaldehydes, and especially from MEK or propionaldehyde.

It is a fourth object of the present invention to provide polymershaving improved adhesion properties, even so good that the need forconventional substrate or polymer melt treatments, such as corona, flameor ozone treatment, to improve the adhesion properties is significantlyreduced or even eliminated.

It is a fifth object to provide a process wherein medium density LDPEpolymers can be prepared with higher ethylene conversions.

It is a sixth object of the present invention to provide a flexibleprocess allowing to provide polymers for blown films, cast films andextrusion coatings in a polymer. density range of 0.923 to 0.935 g/cm³.

It is a seventh object of the present invention to enable theapplication of polymer melt having a higher viscosity onto substrates.

It is an eighth object of the present invention to allow molten polymerweb application with reduced air exposure.

Any other objects of the present invention will become apparent afterconsidering the description herein-below.

The present invention provides ethylene homo or copolymers having adensity of between 0.923 and 0.935 g/cm³, having a molecular weightdistribution M_(w)/M_(n) of between 3 and 10, and comprising from 0.10to 0.50 wt. percent of units derived from a carbonyl group containingcompound, based on the total weight of the homo or copolymer.

According to a further aspect, there is provided a free radicalinitiation polymerization process for the preparation of medium densityethylene polymers or copolymers, comprising reacting ethylene andoptionally one or more comonomers at a high pressure, convenientlybetween 1600 and 4000 kg/cm², and at temperatures of about 150-330° C.in a reactor system consisting of at least one autoclave reactor or of acombination of autoclave and tubular reactors, in the presence of freeradical initiators and a carbonyl group containing compound,characterized in that such amounts of the carbonyl group containingcompound are used so as to provide an ethylene polymer or copolymercomprising 0.15-0.50 wt. percent of carbonyl group containing compoundderived units based on the weight of the total polymer and having adensity of between 0.923 and 0.935 g/cm³.

According to a third aspect, the present invention relates to the use ofa carbonyl group containing chain transfer agent in a polymerpreparation process to increase the adhesion of the polymer melt appliedto a support material.

According to a fourth aspect, the present invention relates to the useof a carbonyl group containing chain transfer agent in a polymerpreparation process to increase the water vapor barrier.

In yet another aspect, the present invention relates to the use of acarbonyl group containing chain transfer agent in polymer preparationprocess for effecting a good and shelf-life stable printability.

In accordance with the present invention it has been found that mediumdensity ethylene homo and copolymers having relatively broad molecularweight distributions can be obtained, which polymers can suitably bedesigned for extrusion coating or cast film applications, bypolymerizing ethylene and optionally a comonomer in the presence ofrelatively high amounts of a carbonyl group containing compound, such asmethyl ethyl ketone or propionaldehyde, in a high pressure autoclave orautoclave-tubular reactor combinations. In this process, relatively highamounts of carbonyl groups derived from, for example, MEK orpropionaldehyde are incorporated in the polymer chains. This high amountof carbonyl groups was found to result in an improved and advantageousperformance behaviour of the medium density polymer product obtained.

In the process of the present invention medium density polymers can beobtained at monomer conversion rates above 20 percent, which issignificantly higher than the conversion rates obtained in conventionaltubular reactor polymerizations which achieve ethylene conversions ofabout 16-18 percent.

Further, it has been found in accordance with the present invention thatthe relationship of medium density and narrow molecular weight forethylene polymers containing ketone or aldehyde derived units asobserved in tubular reactors can be adjusted in the sense that mediumdensity polymers can be obtained with broader molecular weightdistributions, especially by combining an autoclave with a tubularreactor in the polymerization process, allowing the production of broadMWD polymers while maintaining a high density of between 0.923 to 0.935g/cm³.

More in detail, the present invention relates to an ethylene homo orcopolymer having a density of between 0.923 and 0.935 g/cm³, and amolecular weight distribution M_(w)/M_(n) (the ratio of weight averagemolecular weight over number average molecular weight) of between 3 and10, and comprising from 0.10 to 0.50 wt. percent of units derived from acarbonyl group containing compound, such as a ketone and an aldehyde,based on the total weight of the homo or copolymer.

The term ethylene copolymer as used in the present description and theclaims refers to polymers of ethylene and one or more comonomers.Suitable comonomers to be used in the ethylene polymers of the presentinvention and giving the same trends in polymer properties, are, forinstance, ethylenically unsaturated monomers and especially C₃₋₂₀α-olefins, acetylenic compounds, conjugated or nonconjugated dienes,polyenes, carbon monoxide, (meth)acrylic acid, vinyl acetate, and C₂₋₆alkyl acrylates.

The molecular weight distribution of ethylene polymers, ethylenehomopolymers and ethylene α-olefin copolymers, is determined by gelpermeation chromatography(GPC) on a Waters 150C high temperaturechromatographic unit equipped with a differential refractive indexdetector and three columns of mixed porosity. The columns are suppliedby Polymer Laboratories and are commonly packed with pore sizes of 10³,10⁴, 10⁵ and 10⁶ Å. Solutions of the samples (about 0.15 percent byweight) are prepared in 1,2,4-trichlorobenzene stabilised with 200 ppmBHT. The flow rate is 1.0 milliliters per minute, unit operatingtemperature is 140° C. and the injection volume is 200 microliters.

The molecular weight determination with respect to the polymer backboneis deduced by using narrow molecular weight distribution polystyrenestandards (for example Polymer Laboratories) in conjunction with theirelution volumes. The equivalent polyethylene molecular weights aredetermined by using appropriate Mark-Houwink coefficients forpolyethylene and polystyrene (as described by Williams and Ward inJournal of Polymer Science, Polymer Letters, Vol. 6, p. 621, 1968) toderive the following equation:

M _(polyethylene) =a*(M _(polystyene))^(b)

In this equation , a=0.4316 and b=1.0. Weight average molecular weightand number average molecular weight, M_(w) and M_(n) respectively, arecalculated in the usual manner according to the following formula:M_(j)=(Σw_(i)(M_(i) ^(j)))^(j) where w_(i) is the weight fraction of themolecules with the molecular weight M_(i) eluting from the GPC column infraction i and j=1 when calculating M_(w) and j=−1 when calculatingM_(n).

The melt flow index I₂ is determined in accordance with ASTM D-1238,condition (E) (190° C./2.16 kg).

Densities of the polymer products are determined in accordance with ASTMD-792.

The polymers of the invention preferably have a density of between 0.925and 0.930 g/cm³, and preferentially have a molecular weight distributionof between 5 and 9, and most preferably between 6 and 8.

The polymers of the present invention have a weight average molecularweight of between less than 5,000 up to 500,000 and more and preferablybetween 10,000 and 250,000.

In this description and the claims, the term carbonyl group containingcompound is a compound capable to act as a chain transfer agent, whichcompound contains a —C(O)— group and in addition carbon atomssubstituted with hydrogen atoms, wherein a part of the hydrogen atomscan be substituted by inert substituents or moieties. The presence ofunits derived from a carbonyl group containing compound, such as methylethyl ketone or propionaldehyde derived units, can qualitatively andquantitatively be determined using known techniques, for example byusing IR spectroscopy and ¹³C NMR spectroscopy techniques.

The content of units derived from the carbonyl group containingcompound, and especially of methyl ethyl ketone or propionaldehydederived units, is preferably between 0.15 and 0.40, most preferablybetween 0.18 and 0.30 wt. percent based on the total weight of thepolymer. In the most preferred embodiment the polymers of the inventioncontain methyl ethyl ketone derived units.

By using carbonyl group containing compounds as chain transfer agent,carbonyl groups (—C(O)—) are introduced in the polymers formed. Withoutwishing to be restricted to any theory, the present inventors believethat ketones and aldehydes are incorporated in the polymers prepared intwo different ways. When using an aldehyde as carbonyl group containingcompound, the carbon atom of a —C(O)— group is incorporated in thebackbone of the polymer. If a ketone, such as MEK, is used as thecarbonyl group containing compound, a carbon adjacent to the —C(O)—group is incorporated in the polymer backbone. In that case, the polymerprepared will contain pending carbonyl group containing side chains.When using MEK, one will obtain a polymer containing —C(O)—CH₂ sidegroups. As compared with the carbonyl groups derived from aldehydes, thepending side groups are more mobile, and it is believed that thesepending groups have a greater attribution to the advantageous effectsobtained.

In a second aspect, the present invention relates to a free radicalinitiated polymerization process for the preparation of ethylenepolymers or copolymers, comprising reacting ethylene and optionally oneor more comonomers at a high pressure, and at temperatures of about150-330° C. in an autoclave reactor comprising at least two reactionzones or in a combination of autoclave and tubular reactors, in thepresence of free radical initiators and carbonyl group containing chaintransfer agents, preferably MEK or propionaldehyde, wherein such amountsof carbonyl group containing chain transfer agent are used that anethylene polymer or copolymer comprising 0.15-0.30 wt. percent unitsderived from the carbonyl group containing compounds, based on theweight of the total polymer, and having a density of between 0.923 and0.935 g/cm³, is provided.

The process of the present invention is carried out at a high pressure,which means in the context of the present invention that the reactionpressure is at least 1200 kg/cm², conveniently between 1600 and 4000kg/cm².

The process of the present invention is a free radical polymerizationprocess. The type of free radical initiator to be used in the presentprocess is not critical. Free radical initiators that are generally usedfor such processes are oxygen, which is usable in tubular reactors inconventional amounts of between 0.0001 and 0.005 wt. percent drawn tothe weight of polymerizable monomer, and organic peroxides. Preferredinitiators are t-butyl peroxy pivalate, di-t-butyl peroxide, t-butylperoxy acetate and t-butyl peroxy-2-hexanoate or mixtures thereof. Theseorganic peroxy initiators are used in conventional amounts of between0.005 and 0.2 wt. percent drawn to the weight of polymerizable monomers.

The amount of chain transfer agent used in the process of the presentinvention lies between 0.03 and 2.0 percent by weight, and preferablybetween 0.5 and 1.5 wt. percent drawn to the amount of monomerintroduced in the reactor system. Preferably, MEK is used as the chaintransfer agent. MEK has a relatively low chain transfer activity whencompared to propionaldehyde and therefore the amount of carbonyl groupsavailable during the polymerisation process is higher. Further, whenusing a ketone or aldehyde as chain transfer agent, the polymersobtained will have carbonyl groups at the polymer chain. As statedherein-above, it is believed that the pending carbonyl groups have agreater attribution to the advantageous effects obtained. These effectshave been proven in the examples described in this document.

For high pressure, free radical initiated polymerization processes, twobasic types of reactors are known from the prior art. In the first type,an agitated autoclave vessel having one or more reaction zones is used:the autoclave reactor. In the second type, a jacketed tube is used asreactor, which tube has one or more reaction zones: the tubular reactor.The beginning of a reaction zone is defined by the side injection ofeither initiator of reaction, ethylene, telomer, comonomer(s) as well asany combination thereof. The high pressure process of the presentinvention giving polyethylene homo or copolymers having the advantageousproperties as found in accordance with the invention, can be carried outin an autoclave reactor having at least 2 reaction zones or in acombination of an autoclave and a tubular reactor.

In the present process, the pressure in the autoclave reactor, whichcontains more than one reaction zone, is relatively high as comparedwith prior art processes using autoclave reactors, and is preferablybetween 1600 and 3000 kg/cm². In the most preferred embodiment, thereactor pressure is at least 2000 kg/cm², for example 2400 kg/cm². Thehigh pressure values used in the process of the invention have a directeffect on the amount of chain transfer agent, for example MEK orpropionaldehyde, incorporated in the polymer. The higher the reactionpressure is, the more chain transfer agent derived units will beincorporated in the product.

In a preferred embodiment of the process of the invention a combinationof an autoclave comprising at least two reaction zones and aconventional tubular reactor having at least one reaction zone is used.Such a conventional tubular reactor is cooled by an external waterjacket and has at least one injection point for initiator and/ormonomer. Suitable, but not limiting, reactor lengths can be between 500and 1500 meters. The autoclave reactor normally has several injectionpoints for initiator and/or monomer. In this embodiment medium densityethylene homo and copolymers having improved optical properties areobtained. The particular reactor combination used allows conversionrates of above 20 percent, which is significantly higher than theconversion rates obtained for standard tube reactors, which allowconversion rates of about 16-18 percent, expressed as ethyleneconversion, for the production of medium density type of polymers. Thisrelatively low conversion rate of about 16-18 percent is ascribed to thefact that medium density products can be prepared in a tubular reactoronly by lowering the peak temperatures in the reactor considerably,which also leads to a narrow MWD of about 3.

An example of a suitable reactor system is described in for example U.S.Pat. No. 3,913,698, the contents of which are incorporated herein byreference.

When producing medium density polyethylene, the combination of a tubularwith an autoclave reactors offers a broader operating window than thetubular reactor only; the produced polymers can vary from film gradewith a very narrow MWD to coating type resins having a much broader MWD,by enhancing the production in the tube or in the autoclave where eithera minor or a large degree of back mixing is needed. By polymerizingethylene and optionally comonomers in an autoclave reactor, one will geta polymer product having a broad molecular weight distribution, whilethe polymerization in a tubular reactor will give a polymer producthaving a narrow molecular weight distribution; by using combinations ofa tubular and autoclave reactors in series, one can, dependent on thereaction conditions and percentages of monomer polymerized in thereactors design polymer products having all kinds of intermediatemolecular weight distributions. In this way the molecular weightdistribution of polyethylene homo or copolymers can be manipulated withmore flexibility than in a conventional autoclave reactor or in aconventional tubular reactor, while maintaining a high polymer density.

Dependent on the reactor configuration and reaction conditions used, theprocess of the present invention provides polymers that can be groupedinto 3 fields of applications having a medium density and containingrelatively high contents of units derived from the carbonyl groupcontaining compounds used.

When using a combination of an autoclave and a tubular reactor, it ispossible to provide polymers which are usable for blown filmapplications (MWD between 3.0 to 4.0) as class I, cast film applications(MWD between 3.5 to 6.0) as class II, and extrusion coating applications(MWD between 5.0 and 10.0) as class III, depending on the reactionconditions.

Polymers suitable for blown film applications can be obtained bycontrolling the temperature in the two autoclave reaction zones atbetween 150 and 190° C, while the inlet temperature of the monomer feedstreams for both reaction zones is between 50 and 80° C.; and bycontrolling a maximum temperature in the tubular reactor at a value ofbetween 220 and 260° C., while the initiation temperature is between 150and 180° C. The polymer product obtained in such a process, whereinbetween 5 and 7.5 wt. percent monomer, based on the total amount ofmonomer introduced in the reactor, is polymerized in the autoclavereactor and between 13 and 15 wt. percent of the monomer is polymerizedin the tubular reactor, has a melt flow index of between 0.5 and 4.0,and a density of between 0.929 and 0.931 g/cm³. The conversion rate ofthe monomer is about 20-23 wt. percent.

Polymers suitable for: cast film applications can be obtained bycontrolling the temperature in the two autoclave reaction zones atbetween 150 and 190° C., while the inlet temperature of the monomer feedstreams for both reaction zones is between 20 and 60° C.; and bycontrolling a maximum temperature in the tubular reaction zone at avalue of between 250 and 300° C., while the initiation temperature isbetween 170 and 200° C. The polymer product obtained in such a process,wherein between 8 and 10.5 wt. percent monomer, based on the totalamount of monomer introduced in the reactor, is polymerized in theautoclave reactor and between 16 and 18.5 wt. percent of the monomer ispolymerized in the tubular reactor, has a melt flow index of between 1.5and 4.0, and a density of between 0.926 and 0.929 g/cm³. The conversionrate of the monomer is about 26-28 wt. percent.

Polymers suitable for extrusion coating applications can be obtained bycontrolling the temperature in the two autoclave reaction zones atbetween 190 and 220° C., while the inlet temperature of the monomer feedstreams for both reaction zones is between 20 and 60° C.; and bycontrolling the peak temperature in the tubular reaction zone at a valueof between 240 and 290° C., while the initiation temperature is between180 and 220° C. The polymer product obtained in such a process, whereinbetween 10 and 12.5 wt. percent monomer, based on the total amount ofmonomer introduced in the reactor, is polymerized in the autoclavereactor and between 13 and 15 wt. percent of the monomer is polymerizedin the tubular reactor, has a melt flow index of between 3.0 and 12, anda density of between 0.926 and 0.929 g/cm³. The conversion rate of themonomer is about 24-27 wt. percent.

The amount of CO groups built in the polymer chain depends not only onprocess conditions (that is, if conditions used are for class I, II orIII), but mainly depending on the desired MFI of the polymer beingproduced. A film resin produced/belonging to class I may be produced atMelt index 0.3 or up to 4.0 and the amount of chain transfer agentneeded/incorporated will generally be of from 0.10 to 0.50 wt percent ofunits derived from a carbonyl group containing compound and based on thetotal weight of the homo and copolymer), depending on the desired MeltIndex of the polymer being produced.

When using an autoclave reactor containing at least two reaction zones,similar products can be prepared, be it with a lower conversion rate ofthe monomers to be polymerized, by using the reaction conditionsindicated in the previous paragraphs for the autoclave zones.

In a preferred embodiment, the process of the present invention is ahigh pressure process for the production of medium density polyethyleneresins for extrusion coating or cast film applications.

In a very preferred embodiment, the autoclave reactor comprises at leasttwo reaction zones, while the tubular reactor comprises at least onereaction zone. Such a typical reactor configuration makes it possible torun the reaction at relatively low maximum control temperatures ofbetween 150 and 220° C. for the autoclave reactor and of between 230 to290° C. for the tubular reactor in each reaction zone, giving a polymerproduct having a relatively high density, with high conversion rates ofmore than 20 wt. percent monomer.

The present invention uses carbonyl group containing compounds, andespecially ketones or aldehydes, as chain transfer agents in a highconcentration under such reaction conditions that a medium densitypolymer product is obtained and a relative high content of carbonylgroup containing units are incorporated in the polymer product.

Without wishing to be bound to any theory, it is believed that the highamount of carbonyl groups present in the resins of the present inventionresult in the observed improved adhesion of the polymer to substrates,such as paper, aluminum, etc., in coating applications; the peelstrength in such applications considerably increases. The increasedadhesion provides the possibility for higher line speeds in theapplication equipment, with less neck-in of the coating occurring,and/or a smaller air gap. Neck-in is a term known to the person skilledin the art and is defined as one-half of the difference between thewidth of a polymer film at the die opening and the width of the polymerfilm at the nip roll. Together with a reduced neck-in, the edge bead andedge trim will become smaller, as well. The air gap is the distancebetween the die providing a polymer melt and the place where the polymermelt is pressed onto substrate. The air gap controls—at a given appliedcoating weight or thickness, melt temperature and line speed—theexposure time to air. Reducing of the air gap reduces the time foremission and heat loss from the molten polymer to the environment priorto its application onto a substrate. The possibility of using smallerair gaps, makes it possible to run at higher coating line speeds for acertain level of adhesion; or makes it possible to lower the melttemperature of the polymer product used for coating a particularsubstrate which has an advantageous effect on the sensory performance ofthe polymer product obtained by limiting the air exposure time oroxidation time. Hence, the present invention provides a process allowingto coat substrate materials with less oxidized coatings.

Further, the need for pretreatment of the polymer melt or substratesurface in order to improve the adhesion, for example a corona, flame orozone treatment, is reduced. Such known pretreatments give rise todamages of the surface, while in addition generally the level ofemissions of undesirable compounds increases.

More in particular, as compared with a standard extrusion coating resinhaving a Melt Index of 4.1 and polymer density of 0.921 g/cm³ (PG 7004;obtainable from The Dow Chemical Company), it was found that extrusioncoating resins of the present invention had about 30-40 percent betteradhesion properties to paper and aluminum foil, measured atcorresponding coating line speeds.

The resins produced with the process of the present invention have areduced water vapor permeation permitting coating thickness reductions.The reduced water vapor permeation is especially desired in paper andboard based packaging materials, because the structural properties ofpaper and board, such as package rigidity, are sensitive to moisturesorption. As compared with a standard extrusion coating resin such as PG7004 (obtainable from the Dow Chemical Company), it was found that apolymer of the invention having a density of 0.930 g/cm³ and a MFI of3.0 showed a much better water vapor permeation behaviour. The polymerof the invention had a water vapor permeation of about 10.4 (g/m².day;25 micrometer film thickness), while the standard extrusion coatingresin gave a value of about 12.5, which is about 20 percent higher.

The resins produced with the process of the present invention furtherhave improved printing ink adhesion, without need or with a reduced needfor coating surface pretreatments. The increased density providesimproved chill roll release over LDPE, thus permitting the use of highgloss chill rolls required for high quality printing onto the coatedpolymer surfaces.

In further aspects, the present invention hence relates to the use of acarbonyl group containing chain transfer agent in a polymer preparationprocess to increase the adhesion of the polymer prepared to a supportmaterial; to the use of a carbonyl group containing chain transfer agentfor polymers with increased water vapor barrier thus allowing thereduction of polymer coating thickness in articles produced thereof; andto the use of a carbonyl group containing chain transfer agent toenhance printability of the polymer prepared.

Moreover, the increased polymer density provides higher mechanicalstrength and therefore giving the possibility to go to thinner films. Inaddition to that, the corresponding higher melting point and energyrequired to melt, provides extra heat resistance, such as heat exposurein downstream processing (for example drying, sterilization, etc.)

The polymers show an enhanced performance during the fabrication ofarticles thereof. For instance, the polymers produced in accordance withthe present invention have an increased pin-hole resistance controlledby the melt viscosity of the melt when applied onto the substrated inthe extrusion coating nip. Applying a molten polymer at a lowertemperature with less pin holes while maintaining the good substrateadhesion properties described above, improves the water vapor barrier ofthe coating and allows reduced coating thicknesses on sensitivesubstrates such as paper and board.

Further, the invention makes it possible to use carbonyl groupcontaining chain transfer agents to allow the application of molten websof polymer with reduced heat radiation. Lower melt temperatureprocessing offers in addition the possibility to extrusion laminatethinner thermoplastic films with a reduced risk of loss of polymer filmorientation due to heat radiation from the molten polymer web.

The invention is further illustrated by means of the following,non-limiting examples.

EXAMPLE 1

In an LDPE reactor consisting of a two reaction zones stirred autoclave(AC) reactor followed by a two reaction zones tubular reactor, ethylenewas polymerized under the following steady state conditions:

Reactor pressure: 2440 kg/cm²;

Autoclave reactor residence time: around 55 sec

Tubular reactor residence time: around 80 sec

Tert-butyl perpivalate (TBPV) was injected as free radical initiator ineach autoclave reactor zone. At the beginning of the two reactor zonesof the tubular reactor, additional free radical initiator was injected.

Temperature process conditions:

Autoclave top-zone (50 percent ethylene): inlet 90° C., control 170° C.

Autoclave bottom-zone (50 percent ethylene): inlet 70° C., control 170°C.

Tube 1^(st) zone control: 250° C.

Tube 2^(nd) zone control: 252° C.

As chain transfer agent, MEK is used in an amount of 0.8 wt percent inthe reactor make up ethylene feed stream equally divided over both ACreaction zones.

In this continuous process, polyethylene for blown film applications wasobtained with an ethylene conversion rate of 21 percent. The polymerproduct had a MFI of 3.2 and a density of 0.930 g/cm³. The amount ofcarbonyl incorporation was measured using ¹³C NMR and determined to be0.19 wt percent (calculated as MEK groups in the polyethylene chain).

The polymer was measured to have a Mw/Mn value of 3.7

EXAMPLE 2

The polymer product produced in the process described in Example 1 wastested on a extrusion coating film line versus PG 7004, a typical Dowextrusion coating grade prepared in a process wherein a non carbonylbased compound is used as chain transfer agent.

The adhesion behaviour was tested with and without a pretreatment withcorona. More in particular, the paper adhesion of both resins was testedin the Mullen Test. In this test, the test specimen, held betweenannular clamps, is subjected to an increasing pressure by a rubberdiaphragm, which is expanded by hydraulic pressure at controlled rate,until the test specimen ruptures. The pressure reading at that point isrecorded as the bursting strength. Bursting strength is defined as thehydrostatic pressure in kilopascals, or pounds per square inch or psi,required to produce rupture of the material when the pressure isincreased at a controlled constant rate through a rubber diaphragm to acircular area, 30.5 mm (1.2 in.) diameter. The area of the materialunder test is initially flat and held rigid at the circumference but isfree to bulge during the test (TAPPI T 403 om-91). Based on this teststandard an adhesion percentage is calculated, defined as the burststrength measured from the coated side divided by the burst strengthmeasured from the substrate side times 100. The resins were extruded ata set extruder temperature of 290° C. from a coathanger type extrusiondie with a nominal die gap of 0.7 mm, onto 70 g/m² Kraftpaper in anamount of 25 g/m² in parts with in process addition of 40 micronaluminium sheets, using an air gap of 250 mm and varying line speeds inmeters per minute; and at a line speed of 100 m/min, but with varyingair gaps, utilizing a matt chill roll maintained at a temperature of 15to 20° C. Subsequently, the percentage adhesion was determined. Theresults are given in the following tables.

TABLE 1 Paper adhesion (percent) using varying line speeds line speed(meter/min) 100 150 200 250 resin example 1 94 86 79 63 PG 7004 89 70 5039

TABLE 2 Paper adhesion (percent) using varying air gaps air gap (mm) 180250 250(*) 320 resin example 1 93.5 94 100 97 PG 7004 62.5 89 91 96*with corona pretreatment (8 kW)

Further the water vapor transmission was compared for the coatingsapplied at a speed of 100 m/min. The resin of example 1 was found togive a reduction of 30.4 percent as compared with PG 7004 (air gap 180mm) and of 40.5 percent (air gap 250 mm) as presented in table 3.

TABLE 3 Water vapour transmission (WVTR in gr/cm2 at 38° C.) Resin Airgap WVTR percent reduction example 1 180 2.90 30.4 percent PG 7004 1804.17 — example 1 250 2.20 40.5 percent PG 7004 250 3.70 —

In addition, the two types of resins (ex.1 and PG 7004; line speed 100meters/min; 25 g/m²) were applied to aluminium foil (40 μm). Theadhesion of the polymer coating to aluminum foil was measured by peelingthe polymer coating at a 15 mm sample width, at a peeling angle of 180degrees and at a crosshead speed on a tensile tester of 125 mm/min. Theresults are shown in table 4.

TABLE 4 adhesion (N/15 mm) to aluminium foil air gap (mm) 180 250 resinexample 1 3.7 4.6 PG 7004 1.5 2.9 example 1 (corona) 3.8 PG 7004(corona) 2.5

In addition, coefficient of friction to metal was measured for bothtypes of polymers. The results are presented in table 5.

TABLE 5 Coefficient of friction to metal Air gap (mm) 180 mm 250 mmExample 1 0.16 0.18 PG 7004 0.27 0.27

EXAMPLE 3

Example 1 was repeated, yet for the preparation of polyethylene for castfilm applications using the following different steady state conditions:

Temperature process conditions:

Autoclave top-zone: inlet 45° C., control 165° C.

Autoclave bottom-zone: inlet 30° C., control 170° C.

Tube 1^(st) zone control: 275° C.

Tube 2^(nd) zone control: 275° C.

As chain transfer agent, MEK is used in an amount of 0.68 wt percent inthe reactor make up ethylene feed stream, equally divided over both ACreaction zones.

In this continuous process, polyethylene was obtained in an ethyleneconversion rate of 28 percent. The polymer product had a MFI of 2.2, adensity of 0.928 g/cm³ and an Mw/Mn of 5.48. The amount of carbonylincorporation was measured to be in the same level as in example 1.

EXAMPLE 4

The polymer product produced in the process described in Example 3 wastested versus SC 7641 (available from The Dow Chemical Company), atypical Dow cast film grade with a Melt Index of 2.0 and density of0.923 g/cm³ prepared in a process wherein a non carbonyl based compoundis used as chain transfer agent. Table 6 shows a comparison of these twopolymers.

TABLE 6 Mechanical properties and coefficient of friction Resin Example3 SC 7641 Modulus MD 68.7 62.3 Modulus CD 72.3 60.0 Yield TensileStrength MD 4.84 3.91 Yield Tensile Strength CD 3.51 3.13 Film/steelCoeff. of friction 0.67 0.91

EXAMPLE 5

Example 1 was repeated, yet for the preparation of polyethylene forextrusion coating applications using the following different steadystate conditions:

Temperature process conditions:

Autoclave top-zone: inlet 35° C., control 205° C.

Autoclave bottom-zone: inlet 30° C., control 195° C.

Tube 1^(st) zone control: 260° C.

Tube 2^(nd) zone control: 260° C.

As chain transfer agent, MEK is used in an amount of 0.7 wt percent inthe reactor make up ethylene feed stream, equally divided over both ACreaction zones.

In this continuous process, polyethylene was obtained in an ethyleneconversion rate of 25 percent. The polymer product had a MFI of 4.0 anda density of 0.927 g/cm³. The amount of carbonyl incorporation wasmeasured to be in the same level as in example 1.

EXAMPLE 6

The polymer product produced in the process described in Example 5 wastested versus PG 7004 and PG 7008 (available from The Dow ChemicalCompany), both typical Dow extrusion coating grade with a Melt Index of7.7 and a density of 0.918 gr/cm³, prepared in a process wherein a noncarbonyl based chain transfer agent was used as chain transfer agent.

The resins were applied from a coathanger type of extrusion die with anominal die gap of 0.7 mm onto 70 g/m2 Kraftpaper in an amount of 25g/m2 with in-process addition of 40 micron aluminium foil sheets usingair gaps of 180 mm and 250 mm and at a line speed of 100 m/min utilisinga glossy chill roll maintained at a temperature of 15 to 20° C.

At a given wettability described by the similar melt index, tables 7a to7c illustrate a benefit for the example 6 material on foil adhesion overLDPE, based on its improved chill roll release due to density andinherent carbonyl groups establishing improved adhesion also at loweroxidation levels controlled by melt temperature and air gap.

TABLE 7a Foil adhesion 40 micron Al-foil (N/15 mm) Set extrudertemperature 290° C. 310° C. 310° C. Air gap 250 mm 180 mm 250 mm Example6 2.88  3.53 3.65 PG 7004 2.66  3.10 3.36 percent  8.3 13.5  8.6 increase percent percent percent

TABLE 7b Foil adhesion 40 micron Al-foil (N/15 mm) Set extrudertemperature 290° C. 310° C. Example 6 (air gap 180 mm) 1.83 3.53 Example6 (air gap 250 mm) 2.88 3.64 PG 7008 (air gap 250 mm) 2.20 1.83

TABLE 7c Foil adhesion 40 micron Al-foil (N/15 mm) Example 6 (180 mm airgap and 290° C.) 1.83 PG 7008 (250 mm air gap and 320° C.) 1.83

Off taste to water was evaluated according to the following method.Pouches of each polymer coated aluminum foil sample are filled withapproximately 1050 ml of potable water and stored during 24 hours at 30°C. in a dark air heated cabinet. A number of one liter bottlescontaining potable water are stored under identical conditions to beused as reference. After the exposure period, the water exposed to thepolymer bags is evaluated versus the reference samples, for taste andodor properties. An amount of 40 ml of the water exposed is put into a20 centiliter polystyrene cup and covered with a watch glass for twohours before the panel members perform the actual test. All samples arerandomly placed before being offered to the panel members The panelmembers are requested to rate the samples, offered in random order, on ascale of 6 levels starting from 0=no off-flavour/taste up to 5=verystrong off-flavour/taste. After the rating the panel members arerequested to give a forced ranking order to the samples using a scale of4 levels, starting from 1=strongest flavour/taste up to 4=weakestflavour/taste. The results of the rating taste are then evaluatedstatistically.

Tables 8 and 9 show the comparison of both PG 7004 and PG 7008 ersus thepolymer of example 6.

Statistical comparisons with a 95 percent confidence level wereperformed on 25 g/m2 coatings on 40 micron aluminum foil for example 6versus typical extrusion coating grades. Based on the mentioned aluminumfoil adhesion at lower air gaps for the materials of the presentinvention, it is demonstrated in tables 8 and 9 that statisticallysignificant lower off taste to water is achieved.

TABLE 8 Example 6 PG 7004 Set extrusion temperature 310° C. 310° C. Airgap (mm) 180 250 Off taste to water 1.95 2.55 Number of panel members 2222 Duncan range value 0.55 0.55

TABLE 9 Example 6 PG 7008 Set extrusion temperature 290° C. 290° C. Airgap (mm) 180 250 Off taste to water 0.53 1.11 Number of panel members 1919 Duncan range value 0.56 0.56

The table 10 shows the higher heat resistance of the polymers producedin these examples when compared to the standard reference polymers.

TABLE 10 Heat resistance Example 1 PG 7004 Vicat (° C.) 103  95 Meltingtemp. (° C.) 115 111 Energy to Melt (J/g) 149 132 Example 6 PG 7008Vicat (° C.) 100  89 Melting temp. (° C.) 114 107 Energy to Melt (J/g)146 116

The data presented in Table 11 shows the increase in water vapor barrierof the polymers described in example 6 versus the reference polymers.These data were measured on 25 g/m² coated papers produced using aglossy chill roll maintained at a temperature of 38° C. on 70 g/m² Kraftpaper.

TABLE 11 Water Vapour Transmission (WVTR in g/m² during 24 hours) ResinWVTR (g/m²/24 h) percent vs ref. PG 7008 15.9 0.0 (290° C., 100 mpm, 250mm air gap) Example 6 11.5 −27.4 percent (290° C., 100 mpm, 250 mm airgap) Example 6 12.8 −19.1 percent (290° C., 100 mpm, 180 mm air gap) PG7004 14.8 −6.9 percent (290° C., 100 mpm, 250 mm air gap) PG 7004 14.2−10.7 percent (290° C., 100 mpm, 180 mm air gap)

What is claimed is:
 1. An ethylene homo or copolymer having a density ofbetween 0.923 and 0.935 g/cm³, having a molecular weight distributionM_(w)/M_(n) of between 3 and 10, and comprising from 0.10 to 0.50 wt.percent of units derived from a carbonyl group containing compound,based on the total weight of the homo or copolymer.
 2. The polymeraccording to claim 1, wherein the carbonyl group containing compound isa ketone or an aldehyde.
 3. The polymer according to claim 1, whereinthe carbonyl group containing compound is methyl ethyl ketone orpropionaldehyde.
 4. The polymer according to claim 1, having a molecularweight distribution of between 5 and
 8. 5. The polymer according toclaim 1, comprising from 0.15 to 0.30 wt. percent units derived from thecarbonyl group, containing compound.
 6. Polymers according to claim 1permitting melt application in extrusion coating at higher meltviscosity.
 7. Polymers according to claim 1 permitting a meltapplication in extrusion coating at lower temperatures.
 8. Polymersaccording to claim 1 permitting melt application in extrusion coatingwith reduced off-taste to water.
 9. A free-radical initiationpolymerization process for the preparation of medium density ethylenehomo or copolymers, comprising reacting ethylene and optionally one ormore comonomers at a high pressure, between 1200 and 4000 kg/cm², and attemperatures of about 150-330° C. in a reactor system consisting of atleast one autoclave reactor or of a combination of autoclave and tubularreactors, in the presence of free radical initiators and a carbonylgroup containing compound, wherein such amounts of the carbonyl groupcontaining compound are used so as to provide an ethylene homo orcopotymer comprising 0.15-0.50 wt. percent of carbonyl group containingcompound derived units based on the weight of the total polymer andhaving a density of between 0.925 and 0.935 g/cm³ and a molecular weightdistribution M_(w)/M_(n) of between 3 and
 10. 10. The process accordingto claim 9, wherein the carbonyl group containing compound, is used inan amount of between 0.1 and 2.0 percent by weight, based on the totalweight of ethylene and optional comonomers introduced in the reactorsystem.
 11. The process according to claim 10 wherein the carbonyl groupcontaining compound is methyl ethyl ketone.
 12. The process according toclaim 9 wherein the pressure used is between 1200 and 3000 kg/cm². 13.Process according to claim 9, wherein an autoclave reactor combined witha tubular reactor is used as the reactor system.
 14. The processaccording to claim 9, further comprising the step of using a carbonylgroup containing chain transfer agent such that the adhesion of thepolymer prepared to a support material is increased.
 15. The processaccording to claim 9, further comprising the step of using a carbonylgroup containing chain transfer agent such that the polymer prepared hasincreased water vapor barrier properties.
 16. The process according toclaim 9, further comprising the step of using a carbonyl groupcontaining chain transfer agent such that the polymer prepared has areduced the coefficient of friction (COF) to metal.
 17. The processaccording to claim 9, further comprising the step of using a carbonylgroup containing chain transfer agent to improve stiffness of thepolymer prepared.